1. Field of the Invention
The present invention relates to an image recording apparatus for recording image data obtained by a digital fluorographic apparatus or the like.
2. Description of the Related Art
A digital fluorographic apparatus (to be referred to as a DF apparatus hereinafter) involves applying a digital image processing technique to an X-ray diagnosis apparatus. This digital image processing technique converts an output video signal from a TV camera into a digital signal and, if necessary, performs subtraction processing of the digital signal in a real-time manner.
A technique called a digital subtraction angiography (DSA) for injecting a contrast medium into the jugular of a patient and obtaining an angiographic image of a blood vessel is known. More specifically, according to DSA, a contrast medium is injected into the jugular of a patient, and images obtained before and after the contrast medium reaches a blood vessel in a target portion of the patient are subjected to a subtraction to erase an image except for the blood vessel, thereby obtaining a blood vessel image.
It has been attempted to record image data of a blood vessel image or the like processed by the DF apparatus as a real-time permanent file by a recording apparatus such as a digital video tape recorder (digital VTR) or a digital video cassette recorder (digital VCR).
As illustrated by FIG. 1, image data is to be recorded in a VTR, image data from the DF apparatus is transferred to a digital VCR in a real-time manner. The DF image data output from the DF apparatus is gradation-converted using an appropriate display window, thereby obtaining one-frame image data S (512 pixels.times.512 pixels.times.8 bits=2,097,152 bits) of a standard format shown in FIG. 1. A recording area M2 for recording one-frame image data in a digital VCR consists of 512 pixels.times.768 pixels.times.8 bits=3,145,728 bits. The total number of bits of the recording area M2 is larger than the total number of bits of the image data S, so that the image data S can be easily recorded on a digital VCR tape 31.
The following problem is presented as illustrated by FIGS. 1 and 2 when DF image data FD output from the DF apparatus is recorded in a digital VCR without performing gradation conversion. One-frame DF image data FD is 12-bit data, yet the recording area M2 of the digital VCR stores 8-bit data. The number of bits of thus, the DF image data FD is not equal to that of the recording area M2. The DF image data FD consists of 512 pixels.times.512 pixels.times.12 bits=3,145,728 bits, which number of bits is equal to the total number of bits of the recording area M2 of the digital VCR.
However, when additional information such, as a vertical sync signal and a burst signal; is added when the DF image data FD is recorded in the digital VCR, the number of bits required for such additional information is 0.5 to 1. For this reason, 12.5- to 13-bit data is necessary to record the DF image data FD. The total number of bits of the DF image data DF thus exceeds the total number of bits of the recording area M2 of the digital VCR. That is, DF image data DF cannot be recorded in the existing digital VCRs.
The following processing is conventionally proposed to solve the above problems. That is, significant image data from a circle SL is extracted from one-frame DF image data FD (512 pixels.times.512 pixels.times.12 bits) by using a data compressor 12 shown in FIG. 2. Portions except for the circle SL corresponds to 3/4 of one frame are eliminated, and the DF image data FD is compressed. The depth (gradation), i.e., 12 bits, of the compressed image data is segmented into two portions, i.e., 6 bits.times.2 by a pixel bit converter 12a. All the bits of each compressed half, i.e., 512 pixels.times.512 pixels.times.3/4.times.6 bits, are converted from data of 6 bits.times.7 pixels to data of 7 bits.times.6 pixels in units of 6 bits.times.7 pixels. The bit-converted image data is converted into a predetermined standard by an encoder. The converted image data is image data having a special format, and this DF image data D can then be directly recorded in the digital VCR.
Since the format of the DF image data D recorded in the digital VCR has this special format, variable speed reproduction and frame feeding which are provided in the digital VCR as standard functions cannot be performed, and DF image data cannot be easily searched. Therefore, it is inconvenient for a doctor to observe and diagnose an image.